The present invention refers to a method and apparatus for working-up a sample liquid of a pulp and paper industry process water, such as pulp slurry or white water, in an apparatus including a sample flow chamber, a filtrate flow chamber and a filter connecting said sample flow chamber with said filtrate flow chamber.
Due to environmental concerns and the rising cost of chemical additives, paper mills are tending to increasingly close the wet end of the paper machine. This leads to accumulation of dissolved and colloidal substances (DCS), originating from the wood fibers and, as a consequence, process disorders. Deteriorated runability and/or inferior paper quality are frequently experienced. Various chemicals are added to reduce these problems, but the physio-chemical and chemical interactions between DCS and such compounds are complex and often poorly understood. As a result, an overdosage and/or poor utilization of additives is a well known scenario experienced in paper mills.
Considerable laboratory work has been carried out to characterize the chemical nature and behavior of DCS. Such work has provided new and valuable insight into some of the interactions between these components. In order to gain a better knowledge of the dynamics of the chemical interactions, a continuous monitoring of DCS would be of great advantage.
In order to characterize a sample, extraneous material that may hinder or even prevent monitoring and/or analysis equipment from functioning, should be removed. In order to analyze DCS components in e.g. a paper machines white water, varying amounts of fibers and fines present, should be removed.
These fibers (&gt;20 .mu.m) could theoretically be removed from the DCS (&lt;10 .mu.m) by size exclusion. Traditional techniques of filtration, however, show the tendency to remove colloidal substances, as well as, fibers, due to an absorption of lipophilic droplets and colloidal particles in the fiber mat formed on the filter. Successful laboratory methods utilize a centrifuge. A method of continuous centrifugation has been proposed, however this requires a large, expensive and inconvenient decanting centrifuge.
It is an object of the present invention to provide a method and an apparatus, for working-up sample liquid, in which above problems have been minimized.
It is a further object of a preferred embodiment of the present invention to provide a method and apparatus of the above type which allows a continuous monitoring of DCS i process water.
It is a still further object of the preferred embodiment of the present invention to provide a method and an apparatus for continuous on-line fractionation of solid and colloidal substances in process water.
Toward the fulfillment of the above mentioned and other objects, the method and the apparatus according to the present invention are characterized by what is stated in the appended claims.
A working-up of a sample liquid of a pulp and paper industry process water can thereby be achieved according to the present invention by
introducing a flow of sample liquid, including solid substances, such as fibers and filler materials, and dissolved and colloidal substances, through an inlet into a sample flow chamber, PA1 discharging a main flow of such introduced sample liquid through an outlet from the sample flow chamber, PA1 leading from the sample flow chamber through a filter a flow of filtrate into an adjacent filtrate chamber, thereby separating in the filter from a minor portion of said introduced flow of sample liquid, corresponding to &lt;1%, typically &lt;0.1% of the total introduced flow of sample liquid, a predetermined fraction of solid substances, and PA1 preventing a mat of solid substances from building up on the inlet side of the filter by inducing in the sample flow chamber turbulence or a high flow velocity of sample liquid adjacent the filter. PA1 a) Flow of filtrate through filter: PA1 b) Consistency of sample liquid (process water): PA1 c) Flow profile of sample liquid (process water): PA1 d) Effect of the filter itself:
According to a preferred embodiment of the present invention a flow of filtrate, corresponding to less than or about 0.1% of the total flow of sample liquid introduced into the sample flow chamber, is forced to flow through the filter. Thereby only a small amount of solid substance is separated in the filter and added into the remaining main flow of sample liquid being discharged from the sample flow chamber. The added amount of solid substance is that small that it does not noticeably influence the consistency of the main flow of sample liquid.
The sample flow chamber is according to a most preferred embodiment of the invention a sample flow tank, having a filter inserted flush with the bottom thereof and connected to a filtrate or receiver chamber arranged beneath the bottom.
A mixing device, such as an impeller with rotor blades or other similar elements, is arranged in the sample flow tank, for inducing turbulence and preventing build up of solid substances on the filter. Rotor blades are arranged to move relatively close to the filter, e.g. at a distance h=10-25 mm from the filter. One or several baffles may be placed close to the inner side wall of the vertical sample flow tank to increase turbulence.
The sample flow tank can be a relatively high vertical tank, but can also have the form of an oblong trough, as long as a high velocity flow of sample liquid or turbulence is induced therein to prevent build up of solid material on the filter.
A microporous polymer membrane filter, such as a Whatman Cyclopore.TM. Track Etched Membrane made of polycarbonate or polyester or similar other brand polymer membrane, may be inserted in the bottom of the sample flow tank. Such filters usually are very smooth and usually have a thickness of about 7-23 .mu.m, a porosity of about 4-20% and round holes with a pore size of 0.1-12 .mu.m. Of course other filters, such as woven filters and filters with larger pore sizes e.g. up to 70 .mu.m can be utilized.
The flow of sample liquid is forced to flow through the sample flow chamber by a circulating pump arranged upstream or a suction pump downstream of the sample flow chamber in the flow of sample liquid. Thereby a high velocity continuous flow of sample liquid is preferably arranged to flow through the sample flow chamber in order to continuously introduce a representative minor flow of sample liquid through the filter.
A valve or a suction pump is arranged in the flow of filtrate downstream of the filter, for controlling the filtrate flow through the filter. The velocity of the filtrate through the filter should be much less than 10 mm/s, typically about 1 mm/s or even less.
The sample flow chamber is according to another embodiment of the present invention a cylindrical through flow housing, having an inlet in one end and an outlet in its other end and further having at least a segment of its cylindrical wall made of filtration medium. The flow of sample liquid is forced to flow at a high velocity along the filtration medium, for preventing build up of solid substances on the inlet side, i.e. the sample flow chamber side, of the filter.
The present invention provides a new sample work-up method and apparatus for continuous on-line fractionation of pulp and paper process waters. Pulp fibers and large fines are selectively removed from the required analytical sample by utilizing a filtration medium as a size exclusion barrier. A high flow rate of process water is passed across one side of the filter medium with only a relatively small volume of sample passing through the filtration medium. Thus a filter cake and subsequent clogging of the membrane can be prevented as the process water consistency is not significantly reduced by the filtration, due to the small filtrate flow and a fast process water turnover and as fibers are continuously stripped from the filter surface due to turbulence, shear and eddy effects, induced by mechanical turbulence inducing means and/or high flow rates.
In the traditional sense, filtration is the separation of a fluid solids mixture involving the passage of most of the fluid through a porous barrier which retains most of the solid particulate contained in the mixture. Usually the filtrate is transferred through the filter either by pressure applied upstream to the filter medium or by vacuum applied to the filtrate.
The new filtration technique is, however, based on the premises that only the quality of filtrate is critical and that the yield of filtrate is of minor importance. We have found, when working-up samples of pulp slurry, that it is possible to eliminate the buildup of a fiber mat (and subsequent absorption of lipophilic droplets) by increasing the turbulence or the flowrate of pulp slurry adjacent the filter and by substantially decreasing the flow of filtrate compared to traditional filtration.
Following mechanisms are believed to have an positive impact on the new filtration technique, when working-up a good analytical sample from process water, such as pulp slurry:
We have found that the flow velocity of filtrate through the filter medium has a great impact on the flow conditions through the filter medium. Too high flow velocity tends to build up a filter cake in the filter. The flow velocity through e.g. a filter with 10 .mu.m pores, such as Whatman Cyclopore.TM. membrane, should not exceed 10 mm/s, but should typically be about 1 mm/s, preferably even less 10-15 mm/min. High pressure difference easily causes cake formation on filter surface. The pressure difference over the filter should be less than 0.1 bar, preferably negligible. An agitator, such as a magnetic agitator, may be provided on the outlet side of the filter, if a cake of fines or similar substances tends to build up on the outlet side. Also the filtrate flowrate should be as small as possible, e.g. 10-60 ml/min, depending on the effective filter area, may be enough, in order not to cause hold up and integration of filtrate, where momentary variations in filtrate conditions are diluted in large filtrate volumes. PA2 We have found that the factor of process water, e.g. pulp slurry, consistency is important since the quantity of solids present in the water close to the filter has an impact on whether a cake will be formed or not. Local variations in process water consistency over the filter surface may occur resulting in regions of elevated solids content. However, if the process water is uniformly mixed and the quantity of filtrate extracted has negligible influence upon the process water consistency, then such effects are also negligible. Therefore high process water turnover is suggested. PA2 Turbulence, eddy currents and shear forces are invoked in order to strip fibers and solids away from the filter surface. It is essential to have such a sample liquid flow profile that material is continuously being removed from the filter surface by forces within the fluid itself. The shear forces and eddy forces acting close to the filter surface are essential for preventing fibers from either clogging or passing longitudinally through the pores of the filter. The fibers that brush past the filter surface assist additionally by "wiping" the filter surface of fines and other debris that might collect. PA2 Adjacent the filter surface, on the inlet side of the filter, two different forces tend to pull fibers in different directions. Shear forces tend to strip fibers from the filter surface, whereas flow of filtrate tends to pull fibers into the pores of the filter. The filter should therefore preferably be very smooth to prevent fibers from getting caught by the filter surface and be very thin (e.g. &lt;23 .mu.m, preferably even less) to prevent fibers from being permanently stuck in the filter pores/capillaries. If needed (e.g. for ultra thin filters having a large filter area) the filter can be supported from the filtrate side.
A high flowrate of pulp slurry through the sample liquid chamber also provides a continuous relevant sample of liquid in front of the filter.
Low flowrates across filters utilized in the system according to the present invention and accordingly low almost negligible pressure drops over these filters allow very thin filters to be used. Already relatively low pressure drops could mechanically damage ultra thin filters, e.g. of a few micrometer thickness.
Depending on pore size certain fractions of fibers, such as fines, and colloidal substances flow easily through thin filters. Larger fibers may momentarily get trapped with their one end in the pores, the other end still protruding out of the filter surface (the filter being very thin). Such protruding fibers are easily stripped off the filter surface by turbulent flow conditions on the inlet side of the filter. Preferably the distance between adjacent pores in a thin filter is (if practically possible) large enough not to allow one fiber from being simultaneously stuck at its both ends in adjacent pores. A thin filter, if momentarily clogged may easily be regenerated by introducing a high velocity flow or highly turbulent flow of pulp slurry over the filter surface, while the flow of filtrate is temporarily halted.
The filter pore size may be selected according to need. A filter having a pore size of about 0.5 .mu.m may be used to separate fines and colloidal substances, whereas a filter having a pore size of about 70 .mu.m may be used if a filtrate including larger fines is needed.